With a few exceptions, most notably the sponges, animals have differentiated tissues, including a nervous system and muscles, and an internal digestive chamber. Groups with this organization may be called metazoan, though that word is also used for the animals in general.

Until the discovery of protoplasm, and the series of investigations by which it was established that the cell was a fundamental structure essentially alike in both animals and plants (see cytology), there was a vague belief that plants, if they could really be regarded as animated creatures, exhibited at the most a lower grade of life. We know now that in so far as life and living matter can be investigated by science, animals and plants cannot be described as being alive in different degrees. Animals and plants are extremely closely related organisms, alike in their fundamental characters, and each grading into organisms which possess some of the characters of both classes or kingdoms (see protista). The actual boundaries between animals and plants are artificial; they are rather due to the ingenious analysis of the systematist than actually resident in objective nature. The most obvious distinction is that the animal cell-wall is either absent or composed of a nitrogenous material, whereas the plant cell-wall is composed of a carbohydrate material—cellulose. The animal and the plant alike require food to repair waste, to build up new tissue and to provide material which, by chemical change, may liberate the energy which appears in the processes of life. The food is alike in both cases; it consists of water, certain inorganic salts, carbohydrate material and proteid material. Both animals and plants take their water and inorganic salts directly as such. The animal cell can absorb its carbohydrate and proteid food only in the form of carbohydrate and proteid; it is dependent, in fact, on the pre-existence of these organic substances, themselves the products of living matter, and in this respect the animal is essentially a parasite on existing animal and plant life. The plant, on the other hand, if it be a green plant, containing chlorophyll, is capable, in the presence of light, of building up both carbohydrate material and proteid material from inorganic salts; if it be a fungus, devoid of chlorophyll, whilst it is dependent on pre-existing carbohydrate material and is capable of absorbing, like an animal, proteid material as such, it is able to build up its proteid food from material chemically simpler than proteid. On these basal differences are founded most of the characters which make the higher forms of animal and plant life so different. The animal body, if it be composed of many cells, follows a different architectural plan; the compact nature of its food, and the yielding nature of its cell-walls, result in a form of structure consisting essentially of tubular or spherical masses of cells arranged concentrically round the food-cavity. The relatively rigid nature of the plant cell-wall, and the attenuated inorganic food-supply of plants, make possible and necessary a form of growth in which the greatest surface is exposed to the exterior, and thus the plant body is composed of flattened laminae and elongated branching growths. The distinctions between animals and plants are in fact obviously secondary and adaptive, and point clearly towards the conception of a common origin for the two forms of life, a conception which is made still more probable by the existence of many low forms in which the primary differences between animals and plants fade out.

An animal may be defined as a living organism, the protoplasm of which does not secrete a cellulose cell-wall, and which requires for its existence proteid material obtained from the living or dead bodies of existing plants or animals. The common use of the word animal as the equivalent of mammal, as opposed to bird or reptile or fish, is erroneous.

The classification of the animal kingdom is dealt with in the article zoology.

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All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. This may be calcified to form shells, bones, and spicules. During development it forms a relatively flexible framework upon which cells can move about and be reorganized, making a wide variety of complex structures possible.

Most animals are diploid, but polyploid species are known. In sexual reproduction animals produce small motile sperm and large non-motile ova by meiosis. New animals develop from zygotes formed by fusion of the two. Initially the zygote divides to form a hollow sphere called a blastula, which undergoes rearrangement and differentiation. In metazoans, it invaginates to form a gastrula with a digestive chamber. This divides into germ layers that differentiate into tissues: an external ectoderm, internal endoderm, and usually a mesoderm between them.

In contrast, other multicellular organisms such as plants and fungi usually have cells held in place by cell walls, so the sort of complex rearrangements found in animal development can't occur. Instead, they develop by progressive growth.

Animals are generally considered to have evolved from flagellate protozoa. Their closest living relatives are the choanoflagellates, which produce collars identical to those found in certain sponge cells. They also appear closely related to fungi. These form a supergroup called the opisthokonts because their motile cells are propelled by a posterior flagellum, as can be seen in most animal sperm.

The first fossils that might represent animals appear towards the end of the precambrian, and are called Vendian biota. These are difficult to relate to later forms, however, and may even belong to a completely different group. Aside from them, almost every animal phylum with known fossil forms makes a more or less simultaneous appearance during the cambrian period, about 570 million years ago; this massive adaptive radiation is called the Cambrian Explosion.

The sponges (Porifera) were separated from the other animals early on, and are very different. Sponges are sessile and usually feed by drawing in water through pores all over the body, which is supported by a skeleton typically divided into spicules - the cells are differentiated, but not organized into distinct groups.

There are also three problematic phyla - the Rhombozoa, Orthonectida, and Placozoa - that have an unclear position with respect to other animals. When they were first discovered, the Protozoa were considered as an animal phylum or subkingdom, but as they are generally unrelated and often as similar to plants as animals, a new kingdom, the Protista, was devised to hold them.

Aside from these, all animals belong to a monophyletic group called the Metazoa (called the Eumetazoa when the name Metazoa is used for all animals), characterized by a digestive chamber and separate cell layers that differentiate into various tissues. Distinguishing features of the Metazoa include a nervous system and muscles.

The simplest Metazoa are radially symmetric and diploblastic, that is, they have two germ layers. The outer layer (ectoderm) corresponds to the surface of the blastula and the inner layer (endoderm) is formed by cells that migrate into the interior. It then invaginates to form a digestive cavity with a single opening (the archenteron). This form is called a gastrula or planula when it is free-swimming. The Cnidaria (jellyfish, anenomes, corals, etc) are the main diploblastic phylum; the Ctenophora (comb jellies) may also belong here. The Myxozoa, a group of microscopic parasites, have been considered reduced cnidarians but may instead be derived from the Bilateria.

The remaining forms comprise a group called the Bilateria, since they are bilaterally symmetric (at least to some degree), and are triploblastic. The blastula invaginates without filling in first, so the endoderm is simply its inner lining, and the interior then fills in to become a third layer (mesoderm) between the others. Like tissues are grouped into organs. The simplest of such animals are the Platyhelminthes (flatworms), which may be paraphyletic to the higher phyla.

The vast majority of the triploblastic phyla form a group called the Protostomia. These phyla all have a complete digestive tract (including a mouth and an anus), with the mouth developing from the archenteron and the anus arising later. The mesoderm arises as in the flatworms, from a single cell, and then divides to form a mass on each side of the body. Usually there is a hollow space around the gut, called the coelom, arising from a split within the mesoderm, or at least some reduced version thereof (eg a pseudocoelom, where the split occurs between the mesoderm and endoderm, common in microscopic forms).

Some of the main protostome phyla are united by the presence of trochophore larva, which are distinguished by a special pattern of cilia. These make up a group called the Trochozoa, comprising the following:

There are various pseudocoelomate protostomes that are hard to classify because of their small size and reduced structure. The Rotifera and Acanthocephala are closely related to each other and probably belong near the Trochozoa. Other groups include the Gastrotricha, Gnathostomulida, Entoprocta, and Cycliophora. The last was discovered only recently, and as little investigation has been done into the marine world more will probably turn up. Most of these were originally grouped as the phylum Aschelminthes, together with the Nematoda and others, but they do not appear particularly closely related to each other.

The Brachiopoda (lamp shells), Ectoprocta (=Bryozoa, literally moss animals), and Phoronida form a group called the Lophophorata, thanks to the shared presence of a fan of cilia around the mouth called the lophophore. The evolutionary relationships of these forms are very unclear - the group has even been considered among the deuterostomes, and may be paraphyletic. They are most likely related to the Trochozoa, however, and the two are often grouped as the Lophotrochozoa.

The Deuterostomes differ from the Protostomes in various ways. They also have a complete digestive tract, but in this case the archenteron develops into the anus. The mesoderm and coelom do not form in the same way, but rather through evagination of the endoderm called enterocoelic pouching. And, finally, the embryonic cleavage is different. All this suggests that the two lines are separate and monophyletic. The Deuterostomes include:

In Linnaeus' original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the Chordata, whereas the various other forms have been separated out. The above lists represent our current understanding of the group, though there is some variation from source to source.